tests: Handle invalid PDFs
tests: Handle invalid PDFs

If you're missing things like Inkscape, PyMuPDF can fail with errors
like so:

  mupdf: cannot recognize version marker
  mupdf: no objects found
  Traceback (most recent call last):
    ...
  RuntimeError: no objects found

Handle this so the giant traceback doesn't bubble up to the user.

Signed-off-by: Stephen Finucane <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@that.guru>

rst2pdf/rst2pdf,rst2pdf/rst2pdf